An exhaust system of a diesel engine that comprises a NOx adsorber catalyst is capable of adsorbing substantial amounts of oxides of nitrogen (NOx) in engine exhaust gases passing through the exhaust system from the engine. The NOx adsorber catalyst thereby reduces the amount of NOx entering the atmosphere, preventing the trapped NOx from contributing to what might otherwise become smog.
When a NOx adsorber catalyst is present in the exhaust system of a motor vehicle powered by a diesel engine, it is desirable to regenerate the NOx adsorber catalyst from time to time to remove captured NOx so that the catalyst can continue to be effective. Regeneration is typically performed only when prevailing conditions are suitable. The products of regeneration are non-pollutants that are naturally present in the atmosphere.
Naturally occurring petroleum typically contains sulfur in some amount and form, such as in sulfur compounds SO2 and SO3. It remains present to some degree in diesel fuel that results from the refinement of such petroleum. Because sulfur has the capability of poisoning a NOx adsorber catalyst, accumulations of sulfur in a NOx adsorber catalyst need to be removed before they begin to poison the catalyst. Even ultra-low sulfur diesel fuel (fuel having less than 15 ppm sulfur) contains levels of sulfur that are still high enough to collect in, and eventually begin to poison, a NOx adsorber catalyst. Regeneration of a NOx adsorber catalyst to remove adsorbed NOx is typically ineffective to also remove sulfur, and hence de-sulfurization is typically performed by a devoted procedure.
U.S. Pat. No. 6,164,064 describes a process for de-sulfurizing a NOx reservoir catalyst. The process is performed by operating the engine in a manner that elevates the exhaust gas temperature sufficiently to burn off accumulated sulfur, but not high enough to damage the NOx adsorber catalyst.
Typically, a diesel engine runs relatively lean and relatively cool compared to a gasoline engine. Operating a diesel engine in a manner that elevates exhaust gas temperature to levels needed to de-sulfurize a NOx adsorber catalyst is atypical to usual diesel engine operation. Moreover, if the engine is turbocharged, specifications pertaining to the turbocharger may impose an upper temperature limit on exhaust gases that pass through it.
It is believed that an engine, system, and method that can accomplish de-sulfurization of a NOx adsorber catalyst without increasing the temperature of exhaust gases leaving the engine exhaust manifold to those described in U.S. Pat. No. 6,164,064 would be a significant improvement, especially for a turbocharged diesel engine.